The standardization of endodontic instruments is considered important by endodontists since it assures them of optimal selection, uniform availability, convenience and comparability of such endodontic instruments. Over the years, the industry has developed several standards for root canal instruments and in particular for reamers and files. The industry use ISO standard 3630 to define all standard instruments for root canal preparation. Among other things, shapes, profiles, lengths, sizes, acceptable manufacturing tolerances and minimum requirements of mechanical stress are carefully defined, as are other factors such as color codes. The ISO standard 3630 was established by the Technical Committee 106, of the International Standardizations Organization. Pursuant to these standards, an international order numbering system has been developed by which defined sizes for reamers and files are identified. Because such accepted standards have been so ingrained in the practice of endodontics, efforts have been limited in the reexamination of some of the principles upon which these standards have been based. Specifically, the use of reamers and files in endodontics conventionally begins with an instrument having a small diameter. As the endodontist works on a root canal to properly shape and enlarge it, larger instruments are substituted as the endodontist's work progresses. The gradation of increased sizes has become part of the standardization established under the ISO standards. Specifically, the diameters of the cones of successively larger reamers or files are increased under ISO standard sizes, from 0.10 mm to 0.60 mm in uniformly incremental steps of 0.05 mm and in sizes from 0.60 mm to 1.40 mm in uniformly incremental steps of 0.10 mm. These incremental steps are made with acceptable deviations of plus or minus 0.02 mm. Conventionally available standard sets of reamers, files and shapers are all incrementally larger from one to the next larger one by exactly the same absolute dimension. The incremental increases are the same as each larger instrument is used. Thus, the ratio of material removed to the diameter of the canal is much greater at the beginning of the treatment when the canal is small than it is when the larger tools are used towards the end of the reaming or filing process. Accordingly, the necessary delicate feel of the endodontist is impaired by the relatively large incremental steps at the beginning and is less than satisfactory with existing sets of files and reamers. This lack of appropriate feel is particularly apparent in smaller diameter instruments in which, for instance, there is a 50% increase from the 0.10 mm to the 0.15 mm reamer under the ISO standards. This difference is not altogether satisfactory because this change in size does not permit an endodontist a proper control in the deep narrow portions of root canals.
Reference to the use of sets of reamers or files do not consider problems inherent in the absolute diameter differentials between adjacent instruments. For example, U.S. Pat. No. 4,536,159 simply makes reference to the use of sets with uniform 0.05 mm diameter differentials as typically used in preparing the root canal. Other references such as U.S. Pat. Nos. 4,538,989, 4,340,364, 4,332,561 and 4,674,979 either have similarly limited references or do not even discuss the use of endodontic tools such as reamers or files in sets.